It has been demonstrated that chloroquine inhibits the presentation of soluble antigen by accessory cells without affecting antigen binding by such cells. When added subsequent to a few hours incubation with antigen, chloroquine was not inhibitory. Since chloroquine is known to exert its inhibition of cellular function by alkalinization of vesicles that normally become acidified during the process of endocytosis, the present results may indicate that "processing" of soluble antigens by accessory cells may be required in addition to simple antigen presentation to T cells. B cell heterogeneity among those cells capable of being activated by antiimmunoglobulin antibody has been shown by establishing the physical and functional non-identity of such B cells from peripheral blood on the one hand and spleen on the other.